Recently, a fuel cell, which directly converts chemical energy stored in the fuel into electrical energy, has drawn attention as a highly efficient and clean power generating device. The fuel cell has a laminated structure in which a solid electrolyte layer made of an oxide ion conductor is sandwiched between an air electrode (cathode) layer and a fuel electrode (anode) layer from both sides thereof.
At power generation, an oxidant gas (oxygen) is supplied to the air electrode side of the power generation cell, and a fuel gas (H2, CO, CH4 or the like) is supplied to the fuel electrode side thereof, as a reactant gas respectively. Both the air electrode layer and fuel electrode layer are made porous so that the reactant gases can reach the boundary surface of the solid electrolyte layer therebetween.
In the power generation cell, the oxygen supplied to the air electrode layer side reaches near the boundary with the solid electrolyte layer through a pore in the air electrode layer, and there, the oxygen receives an electron from the air electrode layer to be ionized into an oxide ion (O2−). The oxide ion diffusively moves in the solid electrolyte layer toward the fuel electrode layer. When reaching near the boundary with the fuel electrode layer, the oxide ion reacts there with a fuel gas to produce a reaction product (H2O, CO2 and the like), and emits an electron to the fuel electrode layer. The electrons produced by an electrode reaction can be taken out as an electromotive force by an external load on another route.
The flat plate laminated type fuel cell is configured by alternately laminating many power generation cells and separators to form a stack structure, and then by applying a load in the lamination direction from both ends of the stack so that the individual elements of the stack are pressure-bonded and closely appressed to each other.
The separator has a function of electrically connecting the power generation cells to each other and of supplying reactant gases to the power generation cell. The separator includes thereinside a fuel gas passageway which introduces fuel gases to the fuel electrode layer side, and an oxidant gas passageway which introduces oxidant gases to the air electrode layer side.
FIG. 12 illustrates an example of the above separator. The separator 8 is made of a square stainless-steel plate material with a thickness of several mm, and the above described fuel gas passageway 11 and oxidant gas passageway 12 are spirally formed inside the plate material respectively. A circular power generation cell 5 is arranged on the surface of the separator 8 so as to cover the almost entire surface thereof.
In order to increase power generation output of the solid oxide fuel cell, the separators and the power generation cells need to be formed larger, and hence the solid oxide fuel cell needs to be made larger. However, if the size of a power generation cell exceeds a predetermined size (generally, about a diameter of 120 mm), a temperature distribution tends to occur when the power generation cell is operated. The thermal stress induced by the temperature distribution causes thermal distortion, which causes a problem in that the power generation cell (especially, the solid electrolyte layer) tends to be broken.
In light of such circumstances, conventionally there has been proposed a solid oxide fuel cell having a structure made smaller by arranging a plurality of above described power generation cells on the same separator as disclosed in, for example, Patent Documents 1 to 3.
The Patent Document 1 discloses a fuel cell having an internal manifold structure in which a plurality of power generation cells are disposed between the metallic separators and reaction gas (fuel and air) supply openings are disposed in the middle of the individual power generation cell. The supply openings are required to be sealed, and sealing needs to be performed for the number of power generation cells. Therefore, there is a problem in that the seal structure becomes complicated; and if thermal expansion stress during operation causes the seals to be broken, fuel gases are mixed with air, and the heat induced by the mixing causes the power generation cell to be broken.
Unlike Patent Document 1, Patent Document 2 does not provide a power generation cell with a reactant gas supply opening, but is configured such that fuels and air pass through a gas flow channel inside the separator to be ejected toward the center portion of the power generation cell. Therefore, above described problem with the seal structure can be avoided. However, its external manifold structure requires gas supply pipes for fuels and air for each separator, and thereby its reactant gas supply structure becomes extremely complicated. Further, in the separator, the temperature around the air feed section tends to decrease, and the temperature around the fuel feed section tends to increase. Unfortunately, such an uneven temperature in the separator causes the power generation cells to be broken.
Patent Document 3 provides a structure in which a fuel gas preheating pipe and an air preheating pipe are provided in a cavity in the center of the separator through which reactant gases are supplied to the manifold. This structure allows the temperature in the separator to be kept constant. However, since the fuel gas preheating pipe and the air preheating pipe in the center of the separator are cylindrical, there is no gas flowing from each of the laminated power generation cells to the central portion. Therefore, reactant gases flow such that fuels and air are supplied not evenly over the power generation cell but are concentrated in an outer peripheral region thereof. As a result, there is a problem in that the power generation performance reduces. In addition, the gas pipe structure becomes extremely complicated due to its external manifold structure. Therefore, an unnecessary thermal stress is applied to the stack structure during operation and thereby the power generation cell may be broken or the gas pipe may be broken.
As described above, even the solid oxide fuel cells disclosed in Patent Documents 1 to 3 have a problem in that the power generation cell tends to be broken due to thermal stress or the like.
On the other hand, there has been known a power generation cell configured such that a solid electrolyte layer made of an oxide ion conductor is sandwiched between an air electrode layer and a fuel electrode layer, in which, as described above, when air (oxygen) is supplied to the air electrode layer and a fuel gas (H2, CO, CH4) is supplied to the fuel electrode layer, a power generation reaction occurs between the electrodes to obtain an electromotive force.
In such a power generation cell configured above, its electromotive force per unit cell is extremely small amounting to no more than 1 V. Therefore, in general, many power generation cells are laminated into a stack via a separator and a conductive member such as a current collector to obtain substantial cell output.
However, such a flat plate laminated type fuel cell stack is different in temperature between the individual power generation cells in the lamination direction. As shown by C1 in FIG. 13, the stack temperature tends to be high in the middle portion of the stack and low in the end portion of the stack.
This is because Joule heat of the power generation cell is easier to dissipate outside in the end portion of the fuel cell stack than in the middle portion thereof. Further, in the case of a vertically mounted fuel cell stack having a vertical stack lamination direction, the stack's upper portion is heated by an increasing high-temperature exhaust gas. Therefore, the temperature becomes high in the upper end portion and low in the lower end portion.
The power generation cells in a portion having a low temperature are inactive in the electrode reaction, and thus its internal resistance is large, and the power generation performance thereof is lower than that of the power generation cells in the middle portion of the stack having a high temperature.
As described above, the fuel cell stack in which a temperature distribution occurs in the lamination direction thereof has a problem in that efficient power generation cannot be achieved since the total power generation performance is limited by the power generation performance of the power generation cells located in a low temperature portion. The more the number of laminated stacks, the more significant this trend.
Further, the power generation cells in a high temperature portion have good power generation performance, but the components such as power generation cells and separators are more frequently exposed to high temperatures than other components and thus have a problem of being easily deteriorate or broken.
As a technique for equalizing the temperature in the lamination direction of the fuel cell stack, for example, Patent Document 4 is disclosed. According to Patent Document 4, a heat dissipating fin is provided for each separator in a laminated state to improve thermal radiation of the separator, thereby controlling the temperature distribution of the fuel cell stack (lowering the stack temperature by heat dissipation). However, such a control of thermal radiation by heat dissipating fins cannot increase the temperature of, particularly the lower end portion of the stack whose temperature lowers.
Accordingly, the above techniques can reduce the temperature distribution in the lamination direction of the stack to some extent, but the power generation performance of the power generation cells located in the lower end portion of the stack is still kept low. Therefore, the total power generation performance of the fuel cell stack is not satisfiable at all.    Patent Document 1: Japanese Patent Laid-Open No. 06-310164    Patent Document 2: Japanese Patent Laid-Open No. 2002-008683    Patent Document 3: Japanese Patent Laid-Open No. 2003-168469    Patent Document 4: Japanese Patent Laid-Open No. 2004-273140